1. Field of the Invention
This invention relates to a zinc oxide (ZnO) based transparent current spreading and light extraction layer for Group III-Nitride (III-N) Light Emitting Diodes (LEDs), a low temperature aqueous solution method for producing ZnO based transparent current spreading and light extraction layers, and a LED device structure formed by combining a ZnO based transparent current spreading layer with a III-N LED.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Maximizing light extraction is crucial for energy efficient, high brightness light emitting diodes (LEDs). In III-N based LEDs, the typically low electrical conductivity of the p-type layer can result in much higher current injection into the active layers, and thus light generation, near the point of electrical contact. This presents several problems for light extraction and device efficiency. Because most of the light generated is emanating from directly under the opaque metal contact, a large portion is blocked by the contact. Much of the light that is not blocked by the contact intersects the LED surface outside the so called “cone of escape” defined by Snell's Law. Light that is outside of the “cone of escape” undergoes total internal reflection and is thus trapped inside the LED. In addition, the high current density caused by the localized injection near the contact gives rise to heating and other effects that reduce the internal quantum efficiency of the device. The application of a transparent current spreading layer to the surface of an LED can improve these problems in several ways. A transparent current spreading layer distributes current across the surface of the LED, due to a high conductivity, while absorbing very little of the light being generated, due to a high transparency. When possessing certain properties of structure and or refractive index, transparent current spreading layers can also enhance light extraction by reducing the amount of light that is prevented from the escaping the LED due to refraction and reflection at the LED interfaces.
It has been shown that transparent conducting oxide (TCO) films consisting of indium-tin-oxide (ITO) [5,6], ZnO [7], or aluminum-doped-zinc-oxide (AZO) [8], improve external quantum efficiency when applied to the surface of III-N LEDs as transparent current spreading layers. It has also been shown that several methods of TCO layer surface roughening can further improve light extraction from LEDs [9,10]. However, the added cost of depositing and roughening the surface of such TCO layers can largely diminish the commercial advantage of the performance enhancements achieved by these techniques. The current state of the art TCO material for LED current spreading layers, indium-tin-oxide (ITO), has several drawbacks related to cost. First, due to the high price and limited supply of indium, ITO has a high raw material cost which is subject to large fluctuations with demand. In addition, ITO is typically deposited and roughened using techniques that utilize low pressure controlled atmospheres and high power electronics. The expense of the required equipment and energy add significant processing costs to the already high raw materials cost for ITO. Zinc oxide based TCO materials can drastically reduce the cost of raw materials compared to ITO. In addition, ZnO can be synthesized [11] and etched [2,3] using comparatively inexpensive aqueous solution based techniques.
Zinc oxide (ZnO) is an optically transparent, wide band gap semiconductor. A band gap of 3.3 eV, an exciton binding energy of 60 meV, large breakdown strength, and a large saturation velocity have led to interest in ZnO as a possible candidate for use in light emitting devices and other high-power density, high-temperature semiconducting devices. In order to be used in such devices, high quality epitaxial ZnO thin films will typically be required. Many of these applications will also require the ability to produce both n-type and p-type ZnO. Unfortunately, ZnO has a strong tendency for n-type behavior and stable, reliable, and reproducible p-type ZnO has proven extremely difficult to produce. However, the tendency for high n-type conductivity combined with the high optical transparency of ZnO make it very well suited for use as a transparent conductive oxide. Like most inorganic material films used in the semiconductor industry, any ZnO films used are currently produced using vapor phase methods such as molecular beam epitaxy (MBE), pulsed laser deposition (PLD), sputtering, and metal organic chemical vapor deposition (MOCVD). However, it is also possible to produce ZnO films, including epitaxial films, using low temperature aqueous solution methods[11,16,17,18].
Because the general simplicity of the required equipment, along with the low temperatures and atmospheric pressure used, low temperature aqueous solution methods present significant cost advantages over vapor phase deposition techniques. Aqueous solution methods have been used for some time to produce ZnO powders and polycrystalline films, but more recently, it has been shown that epitaxial ZnO films can also be produced using low temperature aqueous solution methods. In general, an epitaxial film will be more transparent and have higher conductivity than a polycrystalline film of the same composition, due to the lack of grain boundaries. However, the current state of the art transparent current spreading layer technology uses a polycrystalline ITO film. Because of the dissimilar crystal structures of III-N materials and ITO, epitaxial film deposition of ITO is generally not possible. Zinc oxide, on the other hand, has the same Wurtzite crystal structure as the III-N materials used in LEDs, making epitaxial growth is possible by numerous deposition methods, including low temperature aqueous solution methods.